How to change material to another material at specified keyframe
You could use a Mix RGB node, plug one texture into the first input and the other into the second input. Then set keyframes for the Mix factor, change it from 0 (first texture) to 1 (second texture). For an instant change insert a keyframe for 0 at frame 19 and another for 1 at frame 20. If you set the 0 at an earlier frame you can make a smooth transition between those textures.
EDIT: Since Alexis mentioned the Mix Shader node in his comment, here something just for completion: If you really meant materials like in the title instead of textures, you can of course create two completely different node setups in one material. And just before the Material Output node you insert a Mix Shader node, connect those two different setups each in one socket and keyframe the Fac value in the Mix Shader node.